While myelotoxicity or bone marrow suppression is a well recognized complication of chemical exposure during cancer chemotherapy, much less attention has been directed towards the effects of environmental agents on the bone marrow. The objective of this study is to examine bone marrow cellularity, pleuripotent hematopoietic stem cells (CFU-S), erythroid precursors, and bone marrow macrophage-granulocyte progenitors (CFU-GM) following exposure to a variety of environmental agents. This study is conducted as part of the immunotoxicology program. Thus, alterations in bone marrow progenitor cells can be correlated with functional alterations in mature cells of the lymphoid system. Studies have been completed to date on over 10 chemicals of environmental concern including polybrominated biphenyl (PBB), tetrachlorodibenzo-p-dioxin (TCDD), diethylstilbestrol (DES), indomethacin, benz(a)pyrene, benz(e)pyrene, and Fryol FR-2. New chemicals being evaluated include those on the NTP Bioassay Program that are potentially immunotoxic as well as others that because of their structure or mechanism of toxicity may aid our understanding of toxicity assessment.